How to Control the Speed of DC Shunt Motors?

DC Shunt Motor Speed Control Methods

In this post, we will discuss about various speed control methods of dc shunt motor.
The speed of the dc shunt motor is given by
N \alpha \frac{Eb}{\phi } \



N \alpha \frac{V - IaRa}{\phi } \
Eb = Back EMF, in volts
Ia = Armature Current, in amps
Ra = Armature Resistance, in ohms
V = Terminal Voltage, in volts
Φ = useful flux per pole, in webers

Thus the speed of shunt motor can be changed by changing any one of the factors mentioned below:

  1. Applied Voltage
  2. Varying the resistance of armature circuit ( Armature control method)
  3. Varying the flux per pole ( Field control method)

Armature Control Method:


  • In this method a variable resistance is inserted in series with the armature coil as shown in the figure.
  • As a result, the back emf is decreased and thus speed of motor is reduced. 
  • Since resistor R is in series with the armature, it carries the full armature current and results in a large power loss in large motors where a considerable speed reduction is required for long periods.
  • This method can only provide speed below normal speed.
  • In this method, speed can be varied upto rated speed. This is the disadvantage of this method. 

Field Control Method:


  • In this method, the external resistance R is connected in series with field winding as shown in figure. By this arrangement, the field current is reduced and the speed is increased.
  • As shown in the above equation, the speed can be varied by varying the value of flux. This can be achieved by using a variable resistor in series with the field winding.
  • This resistor is called as the shunt field regulator.
  • When the value of resistance of the shunt field regulator is increased, the value of the field current(If) will be decreased. 
  • This results in a decrease in the value of flux, , and hence an increase in the speed, since n∝1/Φ
  • In this method, the speed cannot be reduced below  rated speed.

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Thanks for reading about DC shunt motor speed control methods…. Please leave your comments below….

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